This invention relates to mobile cranes of the type known as "rough terrain" cranes in which the operator's cab is mounted on a crane superstructure rotatably carried on the vehicle chassis, and more especially to a steering control system therefor.
Controls for the craning operation are passed through a rotary joint at the rotatable axis of the superstructure and an hydraulically operated steering system is employed so that control is exercized through hydraulic hoses as is the case for boom and crane cable operation.
For use on highways in driving the crane from place to place a fail-safe mechanical steering linkage is required in case of hydraulic power failure; this presents a problem one solution to which has been the provision of rough-terrain cranes having two separate control cabs, one mounted on the chassis with vehicle controls only for use in moving the crane and sufficient duplicated hydraulic controls being provided in a crane control cab for low-speed manoeuvering of the crane in on-site use. Such cranes necessarily have a long wheelbase for permitting rotation of the superstructure with clearance from the chassis control cab.
One of the main difficulties arising in the alternative provision of a mechanical steering linkage engagable for road travel, together with an hydraulic steering system for on-site use, is that the velocity ratio between the steering wheel rotation and steering actuators has a linear relationship in hydraulic operation whereas the ratio between steering wheel rotation and mechanical steering drag link movement is non-linear due to the geometry of a mechanical steering box and drop arm. Proposals for overcoming this difficulty by the use of pressure release valves in the hydraulic system for limiting the build up of excessive pressure engendered when combined with a mechanical linkage, or provision of an hydraulic fluid bypass of the primary hydraulic steering unit are not entirely satisfactory in practice in eliminating differential pressures affecting the steering and also lead to undesirable complications of the hydraulic system.